Metabolic characteristics of retinal cells in tissue culture are under examination. 1. The response of cultured cells of the retinal pigment epithelium to various neurotransmitters and neuromodulators have been examined, especially to vasoactive intestinal peptide (VIP). A specific VIP receptor has been identified on pigment epithelial cell membranes; also, soluble and membrane proteins phosphorylated in response to VIP have been identified. 2. Human retinoblastoma cells in culture can be induced to differentiate in culture into the three major cell types seen in the normal retina, ie, neurons, glia and pigment epithelial cells. Laminin and appropriate other attachment factors appear to play a role in retinoblastoma cell differentiation as well as attachment.